The present invention relates, generally, to methods and compositions for facilitating the processing of hydrogel materials.
More particularly, the present invention provides a method for transiently and controllably enhancing the adhesion of a hydrogel composition to sheets and roll stock, to which the poured hydrogel composition has been applied during processing, to a sufficient degree so that the hydrogel composition can be readily removed following processing with no adverse impact on its textural qualities.
The method of the present invention also works to reduce the subsequent pressure-induced flow of such hydrogel compositions during handling and die-cutting of hydrogel segments from larger sheets and rolls thereof.
In the handling, manipulating, rewinding and, particularly, in the die-cutting of hydrogel sheets, such as those taught in the inventor's prior patent, i.e., U.S. Pat. No. 6,664,301, the innate slipperiness and flexibility of the aqueous hydrogel compositions creates great difficulty in physically manipulating sheets of hydrogel compositions in an intact matter, without distorting, bending, doubling over or an attendant overall loss of control of the dimensional characteristics of the hydrogel sheets and rolls. Further, if the hydrogel sheet is intended for die-cutting, where the fabricated sheet and its release-paper covering (on one or both sides of the sheet) are impacted by a particularly-shaped die, to produce a cut section of the sheet, the very application of the die to the sheet causes the hydrogel composition to squeeze away from the point of impact before the die-cut is made. This results in an undesired and improperly shaped hydrogel piece, thinner than the original sheet, and generally distorted as the result of a full or partial resumption of the original thickness of the hydrogel once the pressure has been relieved.
Furthermore, the hydrogel compositions generally contain 85% or more of water, or water plus such other glycols as glycerin, propylene glycol and related alcohol-based molecules. The hydrogel compositions of particular reference here are those which are prepared from gellants derived from natural plant carbohydrate sources, both terrestrial and aquatic, as well as those elaborated by bacterial cultures. These include gels characterized as guars, xanthans, carrageenans, pectins, starch and starch derivatives, cellulose and cellulose derivatives, konjacs and the like. To prepare the hydrogel compositions, the powdered gellants are often dispersed in cold water or glycol, to which other components of the gel formulation are then added, followed by heating and stirring the mixture to a temperature sufficient for the complex carbohydrate gellants to “open up” so that their chains can interact, and form physico-chemical bonds, with adjacent carbohydrate chains. For this to occur, heating generally takes place at temperatures of upwards of 80° C. When homogeneity is achieved, the mixture is cooled and the gel solidifies, either as a flat sheet induced by the application of a blade positioned over a moving belt bearing the cooling gel, or by introduction of the still-liquid gel solution in shaped forms in which they subsequently cool. The gels produced as large sheets in the former technique are generally those which may benefit from the present invention. Included in the term “sheets” are hydrogel compositions which have been applied to textiles, and in which a fabric such as a scrim has been embedded. Those sheets, either during or following cooling, are generally covered with a protective coating to minimize contamination and moisture loss; the coatings generally being of a paper (cellulosic) base composition. To promote release following processing, the hydrogel sheets are often covered with a film or covering that has at least one surface coated with a natural or synthetic wax composition, the latter often being comprised of a silicone base. Often the moisture of the hydrogel composition will penetrate the paper coating and result in a “wetting” of the cellulosic paper.
The present invention is a result of a search for a method to effectively enhance the physical control of hydrogel compositions, specifically with respect to their handling, manipulation, rewinding and die-cutting. More particularly, the invention provides a method for temporarily adhering hydrogel compositions to certain substrates, during their manufacture, for facilitating the otherwise difficult handling of the hydrogel compositions during their continued processing into a final form. And, once in its final form, the hydrogel composition can be readily detached with no adverse impact on its textural qualities.